1. Filed of the Invention
The present invention relates to apparatus for generating pulse signals.
2. Description of the Related Art
It is necessary to provide a pulse signal in response to the position or speed of a moving object in the field of automatic control or electric or electronic equipment. An electromagnetic pickup is an example of such pulse generators. This electromagnetic pickup comprises a magnetic body, a magnet, and an electrical coil. The magnetic flux varies with movement of the object to be detected to produce a voltage in the electrical coil by the electromagnetic induction. The voltage is used as a pulse signal.
However, the electromagnetic pickup is not suitable in the following field. That is, if the object to be detected moves at very low speeds, the produced voltage is as low as the noise level. If an amplifier is used, the noise is also amplified so that it is necessary to use a filter for removing the noise prior to the amplification. Conversely, the object moves at high speeds, the resulting voltage exceeds the breakdown voltage of the amplifier, thus requiring a limiter. In the case of low speeds, an auxiliary ring is attached to the object to be detected to increase the diameter and thus the peripheral speed. However, this method increases the number of parts and the size of the device. In addition, the timing of rise or fall of the voltage varies with the moving speed of the object to be detected, requiring a complicated signal processor to provide an accurate timing detection. Moreover, the waveform of the voltage varies with the shape of the object to be detected.
Another example is a Hall effect sensor used as a position sensor, angular sensor, or speed sensor. Japanese patent application Kokai No. 2-284082 discloses a Hall effect sensor. This Hall effect sensor comprises a Hall element and a magnet for changing the magnetic flux to the Hall element in response to movement of the object to be detected to provide an electrical signal from the Hall element. However, this type of sensor needs a power source to energize the Hall element. The output electrical signal is a sine wave and cannot produce a sharp pulse signal. If the object to be detected moves at low speeds, the rise of an output voltage is so low that the waveform is truncated. Like the above electromagnetic pickup, it is prone to an external magnetic field and noise and suffers from thermal drift, requiring a complicated processor to provide an accurate detection signal.
Japanese patent application Kokai No. 54-161257 discloses still another type of pulse signal generator. This pulse signal generator comprises a magnetism sensitive element made from a ferromagnetic material so as to have relatively soft and hard portions of magnetic anisotropy, a first magnetic field source for magnetizing the magnetism sensitive element in a positive direction, a second magnetic field source for magnetizing the soft portion of the magnetism sensitive element in a negative direction, a detection coil provided in vicinity of the magnetism sensitive element, and a movable body for interrupting the magnetization of the magnetism sensitive element by the first magnetic field source so that a pulse voltage is produced in the detection coil by the movement of the movable body.
This pulse signal generator is of the powerless type, provides a constant pulse voltage even if the movable object moves at very low speeds, and is resistant against an external magnetic field, thus solving some of the problems of the electromagnetic pickup and Hall effect sensor.
However, this pulse signal generator has the following problems and stands far from practical use. First of all, it needs a movable body with a slit. This movable body cannot be made smaller than the magnet, such as first and second magnetic field sources, and the magnetism sensitive element. Since the slits are provided in the movable body in radial directions, it is necessary to provide a movable body of a large diameter in order to increase the resolution. In addition, the movable body, the magnet, and the magnetism sensitive element should be parallel to each other. The magnet is prone to an external magnetic field or metal to become unstable in operation. It cannot replace the electromagnetic pickup or Hall effect sensor depending on the spacial relationship with the object to be detected. For example, it cannot be disposed to detect directly the teeth of a gear. Moreover, the bias magnetic field applied to the magnetism sensitive element by the second magnetic field source is so unstable that the resulting pulse wave is distorted or the generation timing is not constant, or the pulse wave is prone to noise.